The present invention relates to hydraulic control systems for vehicle automatic transmissions and, more particularly, to a hydraulic control system for a transmission in which a plurality of friction engaging means are selectively engaged by hydraulic pressure, thereby achieving desired gear ratios.
An automatic transmission for a vehicle is known from U.S. Pat. No. 3,754,482, which comprises first, second, third and fourth engaging means. A first shift device is provided in a hydraulic line communicating a source of hydraulic pressure with the first engaging means, for switching supply and discharge of the hydraulic pressure to and from the first engaging means in accordance with running conditions of the vehicle. A second shift device is provided in a hydraulic line communicating the hydraulic pressure source with the second engaging means, for switching supply and discharge of the hydraulic pressure to and from the second engaging means in accordance with the running conditions of the vehicle. A third shift device is provided in a hydraulic line communicating the hydraulic pressure source with the third engaging means, for switching supply and discharge of the hydraulic pressure to and from the third engaging means in accordance with the running conditions of the vehicle. A fourth shift device is provided in a hydraulic line communicating the hydraulic pressure source with the fourth engaging means, for switching supply and discharge of the hydraulic pressure to and from the fourth engaging means in accordance with the running conditions of the vehicle. Two of the first through fourth engaging means are selectively brought to their respective engaged positions, whereby a plurality of gear ratios can be achieved. The arrangement of the automatic transmission disclosed in the above-mentioned U.S. patent is such that supply and discharge of the hydraulic pressure to and from each of the engaging means are switched by ON and OFF operations of a corresponding one of solenoid valves connected respectively to spools of the respective shift devices. Malfunction of the solenoid valves or malfunction of an electronic control system for controlling the solenoid valves would bring one or two engaging means other than a predetermined combination of engaging means to the engaged position or positions simultaneously with movement of the predetermined combination of engaging means to their respective engaged positions. This would cause an inconsistency to occur in a torque transmission path of a gear train in the automatic transmission, so that input and output shafts are locked from rotation or the transmission is damaged. In order to avoid such deficiency, a relay valve is provided in each of the hydraulic lines communicating the hydraulic pressure source with the respective shift devices.
For example, when the solenoid valve is operated, which drives the shift device supplying the hydraulic pressure or line pressure to the 1st gear ratio/reverse engaging means, the line pressure is introduced into the relay valve provided in the hydraulic line between the second shift device and the hydraulic pressure source, to forcibly move the spool of the relay valve to a switching position, thereby intercepting the hydraulic line connecting the hydraulic pressure source to the 2nd gear ratio shift device. This prevents the line pressure from being supplied to the 2nd gear ratio engaging means through the 2nd gear ratio shift device, even if the solenoid valve for driving the 2nd gear ratio shift device is operated due to malfunction or the like of the electronic control system.
Thus, it is possible for the automatic transmission disclosed in the above-mentioned U.S. Pat. No. 3,754,482 to avoid such an inconvenient situation that the hydraulic pressure is simultaneously supplied to the engaging means for achieving their respective gear ratios different from each other so that the gears of the transmission are damaged or locked from rotation.
It is required for the arrangement disclosed in the aforesaid U.S. patent, however, to provide the relay valve in each of the hydraulic lines communicating the hydraulic pressure source with the respective shift devices, in order to achieve the above-mentioned object. By this reason, the hydraulic circuit becomes complicated in structure, and the number of component parts increases, resulting in a rise in the manufacturing cost and in a rise in the probability of occurrence of defects such as valve sticking and the like.
Apart from the above, a technique is known from Japanese Patent Publication No. 48-209 corresponding to U.S. Pat. No. 3,684,066, in which engaging forces acting upon respective engaging means are varied correspondingly to gear ratios. That is, in the low gear ratio in which transmission torque is relatively high, the engaging forces acting on the respective engaging means are raised to increase the torque capacity, while in the high gear ratio in which the transmission torque is relatively low, the engaging forces acting upon the respective engaging means are reduced to bring the torque capacity to a level corresponding to the transmission torque, thereby reducing driving losses of a hydraulic pump.
The arrangement disclosed in the above-mentioned Japanese patent is such that, at shifting of gear ratios, switching is made from relatively high hydraulic pressure supplied to a low speed engaging means to relatively low hydraulic pressure supplied to a high speed engaging means, thereby varying the torque capacity of the engaging means. The arrangement is inconvenient, however, in that shifting shocks occur, because the switching of the hydraulic pressure is made simultaneously with switching of the shift devices, that is, at the early stage of the gear ratio shifting. In order to dissolve the inconvenience, control of the hydraulic pressure supplied to the engaging means for achieving shifting toward the high gear ratio during the gear ratio shifting cannot but become complicated.